A powder of boron nitride (hereinafter called simply BN) has very poor sinterability. Therefore it is a common practice that, as a sintering assistant, a minute amount of at least one oxide such as B.sub.2 O.sub.3, CaO, etc., is added to the BN powder and the mixture is hot pressed into a BN sinter. According to this production process in the prior art, not only is it difficult to obtain a BN sinter of high purity, but it is also almost impossible to obtain a body of complex shape further, the production cost is expensive.
The present invention has been worked out in view of such problems, and has it as an object to provide a BN sinter, suitably of complex shape having high purity and a high mechanical strength made by a process which includes atmospheric pressure sintering.